1. Field of the Invention
The invention relates to a battery monitoring device that monitors a battery unit which connects a plurality of battery blocks in series to each other.
2. Description of Related Art
A battery unit for supplying electric power to a motor, which is a power source, is mounted in a hybrid vehicle or an electric car. A plurality of battery blocks that are connected in series to each other constitute the battery unit, and a plurality of battery cells that are connected in series to each other constitute each of the battery blocks. Each of the battery cells is a secondary battery such as a nickel-hydrogen battery and a lithium-ion battery. Capacities, internal resistances, self-discharge amounts, and the like of the respective battery cells are subjected to variations, and voltages of the respective battery cells are subjected to variation attributable to the above-described variations. In the event of the voltage variation, deterioration of the battery cells is accelerated or the amount of available energy is reduced. An equalization circuit that equalizes battery characteristics between the battery cells in order to remove the voltage variations of the respective battery cells has been proposed (for example, refer to Japanese Patent Application Publication No. 2010-187534 (JP 2010-187534 A) and Japanese Patent Application Publication No. 2014-143853 (JP 2014-143853 A)).
The equalization circuit monitors an output voltage from each of the battery cells. When a battery cell with a high voltage is detected, the equalization circuit attempts to equalize the battery characteristics between the battery cells by performing discharging from that battery cell. In general, the equalization circuit is a circuit that directly connects a switching element and a resistor element to each other, and the corresponding battery cell is discharged when the switching element is turned ON and a current is allowed to flow through the equalization circuit. The equalization circuit, eventually the switching element, is disposed for each of the battery cells and makes an attempt at the equalization by battery cell (for example, refer to JP 2010-187534 A and JP 2014-143853 A).
In recent years, an absolute amount of a variation in the self-discharge amounts of the battery cells is on the increase due to an increase in battery capacity, and a discharge current that is required for the equalization is on the increase as well. In this case, problems arise such as an increase in size, an increase in layout complexity, and an increase in cost regarding the battery monitoring device. For example, in general battery monitoring devices, an IC that outputs an electrical signal for ON/OFF switching to each of the switching elements is disposed for each of the battery blocks. In some cases, the switching element that constitutes the equalization circuit being built into the IC has been proposed. However, when the discharge current for the equalization increases, a heating value of the built-in switching element increases as well, and thus the IC is subjected to an increase in size and cost. In this regard, a configuration for using an external switching element with a large current capacity instead of the switching element built into the IC has been proposed. In this case, however, the external switching element should be prepared for and placed in each of the battery cells. Accordingly, problems arise in the form of an increase in the number of components and an increase in substrate component layout complexity, which, in turn, results in an increase in cost. It is also conceivable that an inter-battery block voltage variation is removed with the equalization circuit being disposed for each of the battery blocks. In other words, it is conceivable that an equalization circuit connected in parallel to the battery block is disposed. In this case, however, an effect attributable to a voltage effect occurs in the event of an equalization circuit control (the switching element of the equalization circuit becoming ON) during battery cell voltage detection depending on equalization circuit connection positions. Accordingly, a problem arises in the form of an inaccurate voltage detection. In other words, the battery cell voltage detection and the battery block equalization control cannot be performed at the same time in some cases.